1. Field of the Invention
The present invention relates generally to methods and systems for non-destructive testing and inspection of wire ropes, wire cables, anchor rods and similar longitudinal cylindrical structures. The present invention relates more specifically to a magnetostrictive sensor probe for use on curved longitudinal cylindrical structural surfaces for guided wave inspection of the structure.
2. Description of the Related Art
Long-range, guided-wave inspection technology is an emerging technology that has the capability of quickly surveying a large volume of a structure for defects and providing comprehensive condition information on the integrity of the structure. Using relatively low-frequency (typically under 200 kHz) guided-waves in the pulse-echo testing mode, this technology performs a 100% volumetric examination of a large area of a structure and detects and locates internal and external defects in the area around a given test position. In above-ground pipelines, for example, a test range of more than 500 feet can be achieved in one direction for detecting 2% to 3% defects from a given test position. In this case, percent refers to the circumferential cross-sectional area of the defect relative to the total pipe wall cross section. In plate-type structures, a test range of more than 30 feet in one direction can be achieved. In wire ropes and cables, a 300 foot longitudinal measurement in one direction can be achieved.
This guided-wave inspection technology, including the magnetostrictive sensor (MsS) technology developed at Southwest Research Institute in San Antonio, Tex., is now widely used for testing piping networks in processing plants such as refineries and chemical plants, as well as for testing plate-type structures such as containment vessels walls and the like. The MsS technology is the only such guided-wave inspection technology that is applied to structures of many different geometries (e.g. pipe, tube, plate, rod, and cable) using a common approach with the same basic instrumentation.
For inspection of elongated structures (such as piping, tubing, rods, or cables), the Magnetostrictive Sensor (MsS) technology uses probes that are generally structured of a length of ribbon cable that encircles the structure's circumference along with a coil adapter that turns the parallel conductors in the ribbon cable into a continuous coil. For inspection of plate-type structures, plate MsS probes in the form of core type probes or flat coil type probes are used. Plate MsS probes in the form of core type probes are disclosed and described in U.S. Pat. No. 6,294,912 entitled Method and Apparatus for Non-Destructive Inspection of Plate Type Ferromagnetic Structures Using Magnetostrictive Techniques, the disclosure of which is incorporated herein in its entirety by reference. Plate magnetostrictive sensor probes in the form of flat coil type probes are disclosed and described in U.S. Pat. No. 6,396,262 entitled Method and Apparatus for Short Term Inspection or Long Term Structural Health Monitoring, the disclosure of which is incorporated herein in its entirety by reference.
Wire ropes and cables are widely used in various structures as important load carrying members. Examples of such longitudinal cylindrical structures include main cables and suspender ropes in suspension bridges, mooring lines in offshore platforms, guy lines for towers, dragline support ropes, and aerial tramway cables. In addition, wire ropes and cables are used in the electric power industry as transmission lines or ground wires.
The magnetostrictive sensor (MsS) technology developed by Southwest Research Institute (SwRI) was originally designed for guided-wave wire rope and cable inspections (U.S. Pat. Nos. 5,456,113 and 5,547,994). More recently, the MsS System is in use for commercial suspension bridge cable inspection services. The existing MsS method generates (and detects) longitudinal guided waves directly in the wires comprising the rope/cable using MsS coils that encircle the rope/cable and DC biasing magnetic circuits (made of either electromagnets or permanent magnets) placed directly on the rope/cable.
The DC biasing magnets necessary for such systems are generally heavy and require careful handling. In addition, the high cost of biasing magnets (a few thousand dollars each) is a drawback for permanent installation of the MsS for long-term structural health monitoring (SHM) of ropes/cables. A further complication exists if the ropes/cables are made of non-ferrous materials or have poor magnetostrictive properties for guided-wave generation and detection. In such cases the ropes/cables may need to be plated or bonded with a layer of magnetostrictive material such as nickel.
Anchor rods are long, slender, tension members that are typically driven into the ground and secured in position by an additional member (called a “cleat”) attached to the rod end. The cleat may be a concrete block, a steel plate, or a steel frame. Typical sizes of anchor rods range from about 7 to 20 feet in length and ¾-2 inches in diameter. Anchor rods are typically made of galvanized structural steel, although a number are made of non-ferrous materials.
During the service life of guyed towers or poles, anchor rods may corrode along their length and thereby lose the capacity to hold the guyed structure, which could lead to catastrophic collapse of the structure. The MsS Technique may be used to inspect these anchor rods from the aboveground exposed section of the anchor rod without requiring costly excavation. Similar to the MsS wire rope/cable inspection, the present method generates longitudinal guided waves directly in the anchor rod using encircling coils and DC biasing magnetostrictive properties. Because of the higher signal amplitudes generated, this invention will also improve the results of anchor rod inspections.
3. Background on the Magnetostrictive Effect
The magnetostrictive effect is a property peculiar to ferromagnetic materials. The magnetostrictive effect refers to the phenomena of physical, dimensional change associated with variations in magnetization. The effect is widely used to make vibrating elements for such things as sonar transducers, hydrophones, and magnetostrictive delay lines for electric signals. The magnetostrictive effect actually describes physical/magnetic interactions that can occur in two directions. The Villari effect occurs when stress waves or mechanical waves within a ferromagnetic material cause abrupt, local dimensional changes in the material which, when they occur within an established magnetic field, can generate a magnetic flux change detectable by a receiving coil in the vicinity. The Joule effect, being the reverse of the Villari effect, occurs when a changing magnetic flux induces a mechanical vibrational motion in a ferromagnetic material through the generation of a mechanical wave or stress wave. Typically, the Joule effect is achieved by passing a current of varying magnitude through a coil placed within a static magnetic field thereby modifying the magnetic field and imparting mechanical waves into a magnetostrictive material present in that field. These mechanical or stress waves then propagate not only through the portion of the magnetostrictive material adjacent to the generating coil but also into and through any further materials in mechanical contact with the magnetostrictive material. In this way, non-ferromagnetic and/or non-magnetostrictive materials can serve as conduits for the mechanical waves or stress waves that can thereafter be measured by directing them through these magnetostrictive “wave guides” placed proximate to the magnetostrictive sensor element.
The advantages of magnetostrictive sensors over other types of vibrational sensors become quite clear when the structure of such sensors is described. All of the components typically utilized in magnetostrictive sensors are temperature, pressure, and environment-resistant in ways that many other types of sensors, such as piezoelectric based sensors, are not. High temperature, permanent magnets, magnetic coils, and ferromagnetic materials are quite easy to produce in a variety of configurations. Further, although evidence from the previous applications of magnetostrictive sensors would indicate the contrary, magnetostrictive sensors are capable of detecting mechanical waves and translating them into signals that are subject to very fine analysis and discrimination.
Examples of efforts that have been made in the past to provide systems and methods for positioning sensors in connection with the inspection of longitudinal cylindrical structures such as pipes and tubes include those disclosed in the following U.S. patents:
U.S. Pat. No. 4,543,528 issued to Baraona on Sep. 24, 1985 entitled Flexible Probe Assembly for Use in Non-Destructive Testing of a Convex Workpiece Surface describes a complicated frame structure that includes a flexible array of sensor heads that are arranged I tension to conform to the pipe when directed against its convex surface. Multiple sensor heads are required in order to provide compliance with the curved surface of the pipe.
U.S. Pat. No. 7,183,674 issued to Goldfine et al. on Feb. 27, 2007 entitled Method for Inspecting a Channel Using a Flexible Sensor describes a system and method for the inspection of components having limited access utilizing a pressurized elastic support structure to position a number of small sensor probes. The probes themselves are not specifically identified as flexible in nature as much as being small enough to be urged against the interior surfaces of a channel using the inflatable elastic structure.
U.S. Pat. No. 7,375,514 issued to Rempt et al. on May 20, 2008 entitled Flexible Handheld MR Scanning Array for Cracks/Flaws describes an NDE device with a sensor probe constructed on a flexible membrane. A frame supports the membrane and incorporates wheels for translation across the surface being inspected. The device is primarily directed to smoothly curved surfaces as may typically be found in aircraft structures.
U.S. Pat. No. 6,967,478 issued to Wayman et al. on Nov. 22, 2005 entitled Pipe Condition Detecting Apparatus describes a fully encircling array of individual sensors for placement in contact with the external wall of a pipeline. The device includes means for inducing and detecting magnetic flux at a location on the pipeline and determining whether changes in magnetic flux reflect changes in the condition of the pipe wall.
U.S. Pat. No. 5,334,937 issued to Peck et al. on Aug. 2, 1994 entitled Magnetic Field Gradient Coil and Assembly describes a magnetic field gradient coil for use in imaging techniques over and within a cylindrical structure. The coil is established through the use of an array of parallel conductors oriented in varying directions around and along the length of the cylindrical structure.
U.S. Pat. No. 6,680,619 issued to Horn on Jan. 20, 2004 entitled Sensoring Device for Monitoring Potential on Corrosion Threatened Objects describes an array of individual sensors positioned on an encircling band about a cylindrical structure such as a pipe. A number of individual cables are connected to the plurality of sensors arranged in a matrix defining measurement points with specifically identified distances.
U.S. Pat. No. 4,784,762 issued to Taliaferro entitled Magnetic Trap describes a method for positioning a Hall Effect sensor on the external surface of a cylindrical pipe. The structure includes a magnetic trap positioned in conjunction with a magnetically transparent sheet on one side of which a magnet is mounted to produce a magnetic field. The Hall Effect sensor is positioned adjacent the magnet to sense the magnetic field.
U.S. Pat. No. 3,568,049 issued to Barton on Mar. 2, 1971 entitled Adjustable Search Shoe for Use in Non-Destructing Testing of Tubular Members describes a sensor structure that includes an object engaging surface that may be mechanically adjusted to change its curvature so as to conform to the wall of the pipe or tubular object under inspection.
U.S. Pat. No. 5,479,099 issued to Jiles et al. on Dec. 26, 1995 entitled Magnetic Inspection Heads Suited for Contoured or Irregular Surfaces describes an arrangement of coils associated with an array of moveable pins within an assembly that is positioned against the curved surface of a pipe or tube. The pins adjust their position according to contact with the external circumference of the pipe and thereby establish a conformed contact surface for the sensor on the magnetic inspection head.
In general, the prior efforts in the field associated with cylindrical structures have been directed to partial circumference sensor structures only where the type of interrogating signal is easily suited to such configurations. That is, none of the previous efforts at partial circumferential orientation have provided suitable sensor adherence structures for use in conjunction with long-range guided-waves. These interrogating waves have heretofore been limited to propagation from sensor structures that circumferentially surround the pipe or tube. In a similar manner, the prior efforts in the field utilizing plate type MsS probes have been limited to individual use on planar structures or within arrays utilizing complex probes and support frame mechanisms, as well as complex multi-sensor operation. None of the previous efforts at either partial circumference sensor structures, or arrays of plate MsS probes, have accomplished the efficient use of such probes at a lower cost and with greater versatility.
It would, therefore, be desirable to have not only a flexible plate type MsS probe that could be used alone or in conjunction with a number of additional probes for investigation of structures having a variety of surface curvatures, but additionally it would be desirable if such a probe was small in size, both for operational optimization and manufacturing efficiency. Once again, while such probe structures could be constructed according to some of the approaches that have been carried out before (in the referenced patent disclosures), such can generally be accomplished only through a significant increase in complexity, both for the probe and the instrumentation associated with its use. The preference would be to simplify the probe's construction and its functionality such that it would be not only simple to utilize in its application (i.e., its placement and positioning on the structure to be investigated) but also in its instrumentation and the ability of standard systems to generate guided-wave interrogation signals sufficient to identify anomalies within the structures at significant distances.
It would also be desirable to have a sensor structure that was economical to manufacture and versatile in its use. The simplicity and ruggedness of its construction would contribute to the requirement for variability in the number of such sensors that might be used in a single structural investigation, while a compact size and flexibility would contribute to the versatility of use in conjunction with a wide range of curved or planar surfaces. To overcome the shortcomings of existing MsS Systems and methods, and to further expand the use of the MsS for long-term SHM of ropes/cables, it would be desirous to have an MsS probe that is lightweight, relatively inexpensive, and is operable independent of the rope/cable material.
In the present invention, systems and methods for constructing and using a plate MsS probe for guided-wave inspection of a variety of longitudinal cylindrical structures are described. The basic probe component configuration is based upon that structure described in the parent Application, U.S. patent application Ser. No. 12/201,989; filed: Aug. 29, 2008; entitled Flexible Plate Magnetostrictive Sensor Probe for Guided-Wave Inspection of Structures; the disclosure of which is incorporated herein in its entirety by reference. The systems and methods are further built upon existing magnetostrictive sensor (MsS) methods and devices, particularly the thin magnetostrictive strip approach (described in U.S. Pat. No. 6,396,262, entitled Method and Apparatus for Short Term Inspection or Long Term Structural Health Monitoring; U.S. Pat. No. 6,429,650, entitled Method and Apparatus Generating and Detecting Torsional Wave Inspection of Pipes and Tubes; and U.S. Pat. No. 6,917,196, also entitled Method and Apparatus Generating and Detecting Torsional Wave Inspection of Pipes and Tubes, the disclosures of which are each incorporated herein in their entirety by reference), and the plate MsS probe (described in U.S. Pat. No. 6,294,912, entitled Method and Apparatus for Nondestructive Inspection of Plate Type Ferromagnetic Structures using Magnetostrictive Techniques, the disclosure of which is incorporated herein in its entirety by reference), but modified and made less expensive and more versatile to fit the specific purposes of the present invention.